In conventional mechanical sewing machines, the amounts of feeding a fabric to be sewn by a feed dog just following values on design (expected value) could scarcely be obtained due to rattlings of feeding mechanisms, irregularities in sizes at composing members and others. Further, said values are not constant for various sewing machines. In this regard, since in said mechanical sewing machines, stitching numbers for forming stitched patterns have been at most 20 stitches, differences between forward and backward feeds, and their accumulated errors have not been practical problems so much.
However, said differences and accumulated errors are serious problems to the sewing machines which electronically store, as pattern signals, amplitude amount of the patterns and feed amount, output the pattern signals to an amplitude control motor and a feed control motor, thereby to control the amplitude control mechanism and the feed mechanism, and produce stitched patterns. Because the sewing machine has unlimited the stitching numbers, it can produce complicated patterns such as alphabet, Japanese letters, abstract patterns or the like, which require much stitching.
Following explanation will be made with reference to FIGS. 1 to 4 for convenience. A stepping motor is used in the sewing machine, and FIG. 1 shows one example between the signal values of the feed amount and the actual feed amount. In the same, the signal values of the feed amount are shown with stepping number of the motor. If the actual feed amount were obtained as the signal value, the feed amount would be drawn as shown with one dotted line. But the actual feed amount is more or less deviated from the signal value of the feed amount by rattlings of the complicated feed mechanism and their accumulation, through manufacturing tolerance of each of the composing members is set within tolerance of the drawings. Although the deviation from the signal value fall within certain ranges in the forward and backward feed amounts, those are made not at fixed rate but irregularly, and irregularities are different in respective sewing machines.
The feed amounts are comparatively stable in the vicinity of intervals shown with, for example, D and E in FIG. 1 with respect to the individual sewing machines, and values near the expected values are obtained. Therefore, some methods use only values around said intervals in order to remove the above problems. However, since the feed amount is included in the large forward and backward ranges, patterns to be formed with those data are limited in themselves.
Another method divides the stitching patterns into a plurality of stitching blocks for making moderate the accumulated errors of the forward and backward feeds of the stitched patterns, as disclosed in the Japanese Patent Application No. 57-174640 by the same applicants.
Complicated patterns of many stitchings have been increased and there have appeared such patterns which could not be limited by the above mentioned two methods. Since the forward and backward feed amounts are not equal as mentioned above, and are different with respect to the individual sewing machines, it is necessary as a matter of fact to provide proper pattern data per each of the sewing machines, but such a matter is quite impossible.
FIGS. 2 to 4 show representative patterns (called as "specific pattern" hereafter) which are easily deformed by the difference between the forward and backward feed amounts and their accumulation.
In them, F arrows are directions of forming stitches by the forward feed, and R arrows are those by the backward feed.
"A" is a buttonhole which is formed with small feed amounts of the feed amounts from the small feed amount of the forward and backward feeds to the large feed amount, and "C" is a darning pattern which is formed with comparatively large feed amounts of the forward and backward feeds. The patterns A.sub.0, B.sub.0, C.sub.0 are produced in standard form. The patterns A.sub.1, B.sub.1, C.sub.1 and A.sub.2, B.sub.2, C.sub.2 are produced as a result of deformation due to the forward feed amount being larger than the backward feed amount, or the forward feed amount being smaller than the backward feed amount.
Since the actual feed amounts are irregular as said above, for example, A, B, C are stitched as A.sub.2, B.sub.1, C.sub.0, and it was not possible to stitch said three patterns regularly by means of the mechanical adjustment.